Cancer cells that slip free of chemotherapy’s deadly embrace sometimes manage to do so because they possess slippery DNA repair proteins. Among the slipperiest are translesion synthesis (TLS) components that lack obvious binding pockets for small molecules. Failing to get a grip on these TLS proteins, small molecule drug candidates perturb the TLS-dependent repair process little or not at all. A newly developed small molecule, however, may be able to exploit a pair of half pockets that form a whole pocket when two TLS proteins combine.
The opportunistic puzzle piece is called JH-RE-06. It was developed by scientists at Duke University and tested in human cancer cell lines and a mouse model of human melanoma. According to the scientists, JH-RE-06 helps preserve the effectiveness of several forms of chemotherapy while also suppressing the ability of cultured cancer cells to mutate in the presence of DNA-damaging drugs. Even better, in the mouse model, JH-RE-06 in combination with cisplatin not only suppressed tumor growth, it also prolonged survival.
These findings appeared June 6 in the journal Cell, in an article titled, “A Small Molecule Targeting Mutagenic Translesion Synthesis Improves Chemotherapy.” The article describes how JH-RE-06 interferes with the normal workings of Rev1, a TLS protein that has eluded so many small molecules it may seem to have a Teflon coating, prompting some scientists to seek alternatives to small molecules, such as genetic disruption.
Sticking with the small molecule approach, the Duke scientists screened 10,000 small molecule compounds and found that JH-RE-06 appeared to do the trick.
“JH-RE-06 disrupts mutagenic TLS by preventing recruitment of mutagenic POL ζ,” the article’s authors wrote. “Remarkably, JH-RE-06 targets a nearly featureless surface of REV1 that interacts with the REV7 subunit of POL ζ. Binding of JH-RE-06 induces REV1 dimerization, which blocks the REV1-REV7 interaction and POL ζ recruitment.”
The researchers used x-ray crystallography to visualize the unexpected interactions between Rev1 and JH-RE-06. They found that when Rev1 interacts with JH-RE-06, Rev1 pairs up or dimerizes with another copy of itself, creating a binding pocket where there wasn’t one before. When Rev1 is locked up in this dimer, it can no longer help cancer cells survive and attain their shape-shifting powers.
JH-RE-06 could enhance the effects of chemotherapeutics such as cisplatin, which are designed to damage DNA, causing the sensitive replication machinery normally tasked with copying each strand to stall. If DNA replication is stalled for too long, cell division halts, and cells die.
The strategy is brutal and effective, even curative in some cases. But long-term, it often fails, as cancer cells figure out a way to proliferate even in the presence of DNA damage.
“The cancer cells often swap out the high-fidelity replication machinery, which usually does the copying, with TLS, a sloppy replacement that covers up the lesions and moves on,” explained the corresponding author of the current study, Pei Zhou, PhD, a professor of biochemistry at Duke University School of Medicine. “As a result, the cells survive, but with mutations in their DNA.
“Chemotherapies are often effective the first time around, but then the cancers mutate and become resistant to that drug, and the next, and the next. It reminds me of Boggarts, those shapeshifting creatures from Harry Potter that morph from one scary thing to another. The beauty of this approach is that you essentially freeze the Boggart in its current form, so you can kill it off for good.”
When Zhou and colleagues tested JH-RE-06 in the mouse model, they were encouraged by what they observed. “Co-administration of JH-RE-06 with cisplatin suppresses the growth of xenograft human melanomas in mice,” they wrote in Cell, “establishing a framework for developing TLS inhibitors as a novel class of chemotherapy adjuvants.”
Senior study co-author Jiyong Hong, PhD, a professor of chemistry at Duke, said that Zhou’s team is currently creating versions of JH-RE-06 that have enhanced pharmacologic properties that could make it an even more attractive drug. “This is a great proof of principle that it is possible to target this protein, but we have a lot of work to do to turn this lead compound into a viable candidate that we can take to the clinic.”